Water tube boiler

ABSTRACT

A water tube boiler comprises a furnace having a burner and a vertical flue gas stack located to one side of the furnace. A convection tube assembly is arranged in the stack for the recovery of heat from flue gases passing through the flue gas stack. With the object of facilitating any repair work needing to be performed on the convection tube assembly, the tube assembly comprises by a plurality of parallel rows of vertical convection tubes which are provided externally with surface enlarging elements e.g. in the form of pins, along part of the height of each tube. The tubes in each row are joined together to form a coherent flat unit with the aid of a respective upper, horizontal header to which all convection tubes in the row are connected at their upper ends, and a respective lower horizontal manifold to which the bottom ends of the tubes in the row are connected. The headers of the various units are connected individually to a steam drum of the boiler and their manifolds are connected individually to a stuff box in the boiler. The heights of the headers and manifolds for adjacent flat tube units across the stack respectively alternate in height vertically.

BACKGROUND OF THE INVENTION

The present invention relates to a water tube boiler, particularly ofthe type comprising a furnace having at least one burner located in itsupper part to generate a generally downwardly directed flame into thefurnace, a substantially vertical flue gas stack located close to oneside of the hearth and having an inlet for the flue gases at its lowerpart which is situated in a lower part of the furnace, an outlet forflue gases located at its upper part, a convection tube assemblyarranged in the stack for the recovery of heat from flue gases passingthrough the flue gas stack.

In conventional steam boilers of this type, the convection tube assemblyarranged in the flue gas stack generally consists of a plurality oflayers of substantially horizontal tubes located one above the other.Occasionally substantially vertical convection tubes have been used.However, the efficiency of the convection part of that boiler has beencomparatively low. Both types of known constructions have been found inpractice to be highly unfavourable regarding the extent of anddifficulty in performing the operations required to repair theconvection tube assembly after the appearance of a leak in any tubes.

SUMMARY OF THE INVENTION

The object of the invention is to effect an improved water tube boilerof the type described which enables repair work on the convection tubeassembly to be performed in a considerably simpler and quicker mannerthan was previously possible.

The boiler according to the invention has its convection tube assemblycomprising a plurality of parallel rows of substantially verticalconvection tubes extending along a substantial portion of the length ofthe flue gas stack and the tubes are provided externally with surfaceenlarging elements. The tubes in each row are joined together to form aflat coherent unit with the aid of an upper, substantially horizontalheader to which the upper ends of all convection tubes in the row areconnected, and a lower, substantially horizontal manifold to which thelower ends of the convection tubes are connected. The headers of thevarious units are connected individually to a steam drum of the boiler,and their manifolds are connected individually to a stuff box in theboiler.

By this construction of the convection tube assembly, when a leak occursin a convection tube, the flat unit comprising a row of convection tubeswhich includes the damaged tube can be cut free and removed from theflue gas stack. The damaged tube can then easily be repaired or replacedby a new tube. The flat unit of convection tubes can be cut free bycutting each of the two tubes connecting the unit's header with thesteam drum and its manifold to the stuff box of the boiler. Furthermore,providing the convection tubes with external surface enlarging elementsavoids the drawback of previously known steam boilers havingsubstantially vertical convection tubes, i.e. low efficiency in theconvection part of the boiler, and instead enables high efficiencythere.

With the object of reducing the resistance of the substantiallyhorizontal headers and manifolds in the convection tube units to theupwardly directed flow of flue gases in the stack, both headers andmanifolds may be arranged mutually displaced in a substantially verticaldirection between adjacent units. The headers and manifolds are suitablyarranged displaced in alternate upward and downward directions betweenthe various units.

To facilitate advantageous placement of suitable mechanical supports forthe convection tubes and equipment necessary for soot blasting, theconvection tubes may be provided externally with surface enlargingelements only along selected portions of their length. In this case,they may suitably be provided between such parts with parts which arefree from external surface enlarging elements.

The distance of diversions between the convection tubes may suitably bethe same within each unit and between the various units. This ensuresuniform distribution of the convection tubes throughout the entirecross-sectional area of the flue gas stack.

In a preferred embodiment of the invention the surface enlargingelements are formed by pins applied on the convection tubes andprojecting generally radially from them. If the distance of divisionsbetween the convection tubes as mentioned above is the same within eachunit as it is between the various units, the outermost ends of the pinson each convection tube along at least parts of the tube length may belocated along the sides of a described rectangle. Each convection tubemay then be provided with pins directed toward the corners of thedescribed rectangle, those pins being longer than the pins locatedbetween them. Preferably, at least some of the pins on each convectiontube are bent in such a manner that they extend obliquely towards theupper end of the tube.

The invention is described in detail below with reference to theaccompanying drawings in which

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 exemplary shows a side view, in section, of a steam boileraccording to one embodiment of the invention,

FIG. 2 shows a rear end view of the boiler, partly in section,

FIG. 3 shows a horizontal projection of the boiler, partly in section,

FIG. 4 shows a detail on a larger scale, revealing a central part of oneof a plurality of convection tube units arranged in the vertical fluegas stack of a boiler,

FIG. 5 shows a detail in section along the line V--V in FIG. 4,

FIG. 6 shows a detail, similar to FIG. 4, but revealing a lower portionof a convection tube unit, and

FIGS. 7 and 8 show details on an even larger scale, revealing moreclearly the design of two different types of surface enlarging elementsapplied on the convection tubes.

DESCRIPTION OF A PREFERRED EMBODIMENT

The steam boiler 10 constitutes a water tube boiler having asubstantially rectangular shape in both a horizontal and a verticalprojection. A furnace 11 is located in a front part of the boiler, and avertical flue gas stack 12 is located in a rear part. A stack isprovided at its upper end with a flue gas outlet 13. The boiler 10 ismounted on a foundation 14 on which it rests supported partly by anumber of supports 15 carrying stuff boxes 16 and 17 which are locatedat the lower end of the boiler and partly by the lower ends of twodowners 18, the upper ends of which support the steam drum 19 of theboiler.

The furnace 11 is limited laterally, forward and rearward by panel tubewalls consisting of rows of equally spaced tubes 20 connected togetherby means of intermediate waist plates 21. The flue gas stack 12 is alsolimited laterally, forward and rearward by similar panel tube walls. Theexterior of the boiler lo is covered with a layer 22 of heat insulatingmaterial and then with a layer 23 of sheet metal.

The panel tubes 20 located around the hearth 11 are connected by theirupper ends to the steam drum 19 by way of header boxes 24 and 25, and bytheir lower ends to the previously mentioned stuff boxes 16 and 17,respectively. These stuff boxes communicate with the two downers 18 toreceive water flowing down through the downers from the steam drum 19and distribute it to the various panel tubes 20 through which the water,being partially vaporized, flows up by auto-circulation to the headerboxes 24 and 25, respectively, and from there to the steam drum 19.

A burner for gas or oil is located at the upper end of the furnace 11.It is arranged to produce a generally downwardly directed flame 27,indicated by broken lines, into the furnace. The flue gases producedupon combustion of the fuel consisting of gas or oil are withdrawn fromthe hearth 11 to the flue gas stack 12 through a flue gas inlet 28,located in the lower part of the stack and having its orifice in thehearth. The flue gases then pass up through the flue gas stack to theoutlet 13 located at the upper end of the stack.

The flue gas inlet 28 is formed by a large number of gap openings 29between adjacent tubes 20 in the lower part of the partition 30 in theform of a panel tube wall located between the furnace 11 and the fluegas stack 12. These openings 29 are provided by omitting the waistplates 21 normally present between the tubes 20 in the lower part of thewall 30. Furthermore the tubes 20 within this part of the wall 30 havebeen bent so that they are displaced alternately a short distanceforward and a short distance backward in relation to the parts of thetubes located in part of the wall 30 higher up, in order to increase thewidth of the intermediate openings 29.

For the purpose of recovering heat from the flue gases passing throughthe stack 12, an assembly of vertical convection tubes 31 is providedinside the stack. They extend along a considerable length of the fluegas stack and are arranged in a plurality of parallel rows. These tubes31 are provided along parts of their length which are spaced from eachother with external surface enlarging elements in the form of pins 32which project in generally radial direction from the tubes, whereasother parts of the tubes, e.g. the intermediate parts designated D and Ein FIG. 1, lack such surface enlarging elements. The purpose of thesurface enlarging elements formed by the pins 32 is to increase heatabsorption in the tubes 32 and thereby improve the efficiency of in theconvection tube assembly formed thereby in the flue gas channel.

The convection tubes 31 form a flat coherent unit within each row. Allof the tubes in one row are connected together at their upper ends bymeans of an upper horizontal header 33 and at their lower ends by meansof a lower horizontal manifold 34. A panel tube 20 located opposite eachconvection tube unit in the panel tube wall situated at the rear of theflue gas stack 12 is also connected by its upper and lower end,respectively, to the header 33 and manifold 34 of said unit.

The headers 33 of the various convection tube units are connectedindividually to the steam drum 19, each by its own connecting pipe 35,while their manifolds 34 are connected individually to the stuff box 16,each by its own connecting pipe 36. Each convection tube unit will thusform a flow path, separated from the other units, between the stuff box16 and steam drum 19, along which water can flow by means ofautocirculation while absorbing heat from the flue gases passing throughthe flue gas stack 12.

The division of the convection tubes 31 into several parallel flat unitsas described above enables repair work or the exchange of a faulty tubeto be carried out in a considerably simpler manner than has previouslybeen possible. Once the appropriate part of the sheet metal casing 23and the layer 22 of insulating material beneath the casing have beenremoved from the rear of the flue gas stack 12, it is easy to cut freethe unit which includes the faulty tube and remove this unit for repairor to replace the tube. The unit can then be reinserted in its place inthe flue gas stack 12 and welded to the steam drum 19 and the stuff box16 by way of their connecting pipes 35 and 36, respectively.

As can be seen in FIGS. 1 and 2, both the headers 33 and the manifolds34 are arranged displaced alternately in upward and downward directionbetween the various convection tube units. The flow resistance of thesepipes to the flue gases passing through the flue gas stack 12 is thusreduced.

The sections of the convection tubes 31 designated A, B and C in FIGS. 1and 2 which are provided with pins 32 projecting from them to formsurface enlarging elements, as indicated only schematically in saidfigures, may be constructed in the following manner.

If the tubes 31 are arranged as shown in FIGS. 4 and 5 with equalspacing within each flat unit formed by a row of such tubes, and betweenthe various units, the pins 32 along the two upper sections A and B ofthe tubes 31 may be constructed as shown in FIGS. 4 and 5. Here theouter ends of the pins 32, seen in the longitudinal direction of thetube 31, are situated along the sides of a described rectangle, whereineach tube 31 has four pins 32' of the type shown more clearly in FIG. 8directed toward the corners of the rectangle and the pins 31' are longerthan the pins 32" situated between them, the pins 32" being more clearlyin FIG. 7. Both the pins 32' and the pins 32" are bent so that theyextend obliquely from the ends attached to the tube 31 toward the upperend of said tube. The shorter pins 32", however, are bent considerablymore than the longer pins 32'.

As shown in FIG. 6, within the lower section C of each tube 31, the pins32 may decrease gradually in length toward the lower end of the tube 31and extend radially outward from the tube in a direction perpendicularthereto. The reason for the special shape of the pins 32 located withinsection C is that the flue gases have an extremely high temperature whenthey reach this section and it is therefore suitably to give the pins asuitable length for this high temperature to ensure that they will notbe subjected to injurious heating by the flue gases.

Within sections D end E of the convection tubes 31 located betweensections A, B and C, and also within the lowermost section located undersection C, soot blasting pipes 37 protrude into the flue gas stack 12.These pipes 37 are provided with a plurality of openings 38 distributedalong their length and around their circumference, through which steamcan be blown in between the tubes 31 from steam inlets 39 situated atthe outer ends of the pipes.

A bracing means 40, shown more clearly in FIG. 4, is also providedwithin section E for mechanical connection of the individual tubes 31 ineach unit to each other at a point situated approximately centrallybetween the tube ends along their length.

Finally, a superheater consisting of a plurality of horizontal pipeloops 41 is provided in the lowermost section of the convection tubeassembly.

The invention is not limited to the embodiments described above andillustrated in the drawings. Many other embodiments are feasible withinthe scope of the invention. For example it may be mentioned that thepins 32 arranged on the convection tubes may be replaced by other typesof suitable enlarging elements.

We claim:
 1. A water tube boiler comprisinga furnace having a burner forgenerating a flame in the furnace, lateral sides around the furnace, asubstantially vertical, flue gas stack located close to one of the sidesof the furnace; an inlet to the stack from the furnace for flue gases,the inlet located in the lower part of the furnace and the lower part ofthe stack; an outlet for flue gases located at the upper part of thestack; a convection tube assembly is arranged in and extendingvertically in the stack for recovering heat from the flue gases passingthrough the stack, the convection tube assembly comprising a pluralityof parallel rows of substantially vertical convection tubes, the tubesextending along a substantial portion of the height of the stack; therespective tubes in each row being joined together to form a flatcoherent unit, a respective upper, substantially horizontal header foreach row of tubes and the header being connected at the upper ends ofthe tubes in the row, a respective lower and substantially horizontalmanifold to which the lower ends of the tubes are connected; a steamdrum of the boiler to which the headers of the units are connected; astuff box in the boiler to which the manifolds of the units areconnected; and surface enlarging elements provided on the exteriors ofthe tubes along at least a portion of the height of the tubes in thestack.
 2. The boiler of claim 1, wherein the furnace has an upper partin which the burner is located and the burner generating a generallydownwardly directed flame into the furnace.
 3. The boiler of claim 1,wherein the headers are individually connected to the steam drum and themanifolds are individually connected to the stuff box.
 4. The boiler ofclaim 1, wherein in alternate units across the stack, both of the headerand the manifold for a first one of the units is vertically upward withrespect to the adjacent units and the header and the manifold for asecond one of the units is vertically downward with respect to theadjacent units.
 5. The boiler of claim 4, wherein in alternate tubeunits across the stack, the headers and manifolds are alternately spacedupward in one unit and downward in the next adjacent unit.
 6. The boilerof claim 1, wherein the surface enlarging elements are provided on theconvection tubes at selected ones of the portions along the height ofthe tubes and not at other portions along the height.
 7. The boiler ofclaim 6, wherein the convection tubes are in a number of sections, withsections provided with the surface enlarging elements alternating withsections that are free of the surface enlarging elements.
 8. The boilerof claim 1, wherein the distance of the divisions between the convectiontubes is the same within each unit and between the various units.
 9. Theboiler of claim 1, wherein the surface enlarging elements comprise pinsformed on the exterior of the convection tubes and projecting radiallytherefrom.
 10. The boiler of claim 9, wherein the convection tubes arein a number of sections, with sections provided with the surfaceenlarging elements alternating with sections that are free of thesurface enlarging elements.
 11. The boiler of claim 10, wherein the pinson each tube project outwardly from the tube a distance so as to definea rectangle shape defined by the ends of the pins from the tube.
 12. Theboiler of claim 11, where on the parts of the tube having the pins, someof the pins project toward the corners of the rectangle and the pinsdirected to the corner project longer than the pins located between thepins projecting toward the corners of the rectangle.
 13. The boiler ofclaim 12, wherein at least some of the pins on each convection tube arebent so that they extend obliquely toward the upper end of the tube. 14.The boiler of claim 11, wherein at least some of the pins in eachconvection tube are bent so that they extend obliquely toward the upperend of the tube.
 15. The boiler of claim 9, wherein at least some of thepins in each convection tube are bent so that they extend obliquelytoward the upper end of the tube.